Photoelectricity And Valley Properties Of Two Dimensional Materials

In recent years,two-dimensional materials have became research hotspots in the fields of physics,chemistry and materials science,etc.Their unique crystal structure and excellent photoelectric,mechanical,thermodynamic,and magnetic properties have attracted the interest of researchers.In 2004,the successful exfoliation of graphene with special structure and properties arouses people's interest in the research of two-dimensional nanomaterials.Gradually,abundant new two-dimensional materials which possess similar structure with graphene and diverse functions are found,such as black phosphorus,transition metal dichalcogenides,transition metal halide,hexagonal boron nitride,metal organic framework,etc.Their emergence expand the family of two-dimensional materials.The two-dimensional materials have excellent physical and chemical properties because of its unique structure.In addition,there are a large number of materials with rich properties,which enable us to choose specific materials to meet the specific development needs.Their applications in electronics,photoelectron,sensors,catalysis,gas separation,energy storage and conversion,biomedicine as well as other fields have been widely explored.In addition,it is found that two-dimensional materials can be stacked then form a variety of heterostructures,that is,van der Waals heterostructures.By constructing heterostructures,not only abundant crystal structures can be obtained,but also the electronic,optical,thermodynamic and magnetic properties of materials can be optimized.Then,the devices related with heterostructures can work better.The heterostructures related devices,including transistors,information storage devices,photodetectors and photovoltaic devices and so on have attracted extensive attention.For a long time,people are mainly foucs on the research of the charge and spin degrees of freedom of two-dimensional materials.Based on this,a large number of charge and spin devices have been manufactured,which greatly promotes the development of information technology.With the develepment of the research,a new type of freedom,namely valley degree of freedom has been found.This degree of freedom can also be easily detected and polarized.The study of valley degrees of freedom gave birth to valley electronics.The related research of valleytronics mainly uses valley degrees of freedom to encode and process information.At present,transition metal dichalcogenidesare are the most studied energy valley materials.They have direct band gap and inversion symmetry breaking structure,and it's Brillouin zone has a series of degenerate energy valleys.Take transition metal dichalcogenides as an example,under certain external conditions,bloch electrons can have non-zero Berry curvature and orbital magnetic moment,resulting in the valley Hall effect.At present,the most important point to realize the reasonable application of valley degree of freedom is to break valley degeneracy then obtain valley polarization.Although valley polarization can be achieved by optical,electrical and magnetic strategies,the valley polarization value obtained is relatively small.Besides,the factors which can affect valley polarization and the mechanism of valley polarization are still unclear.Therefore,to find suitable valley materials and appropriate control methods then obtain stable and large valley polarization are still the research hotspots of valleytronics.In this paper,we systematically studied the photoelectric,spin and valley polarization properties of two-dimensional materials via first-principles calculations methods.It is found that the photonics and electronics properties of two-dimensional materials can be effectively tuned by introducing defects.The spin polarization of two-dimensional materials can be achieved by doping transition metal atoms.Simultaneously,the materials can maintain both semiconductor and ferromagnetic properties.In addition,by doping or adsorbing transition metal atoms and building magnetic heterostructures,we can obtain large valley polarization in two dimensional materials.In addition,the electronic structure and valley polarization of the material can be effectively optimized by applying strain.The research contents of this paper are as follows:1.Optical properties of monolayer PdSe₂ by introducing defects.PdSe₂ is a special two-dimensional material different from Mo S₂.It has a rare pentagonal structure,stable structure,suitable adjustable band gap,and excellent light absorption properties.By introducing Pd or Se defects,the electronic structure of the monolayer PdSe₂ changes significantly,e.g.,band gap decreases and intermediate states appeared in the band gap.The optical properties of materials are directly related to electronic properties,hence its optical constants,including dielectric constant,absorption spectrum,refractive index,reflectivity,and electron energy loss spectrum,have all significantly changed.In particular,the light absorption region becomes wider,which will help the application of PdSe₂ in optoelectronic devices.In addition,the difference peaks caucsed by different defects can be used to identify different types of defects.2.Effects of doping transition metal atoms on the spin polarization properties of PdSe₂.PdSe₂ has strong spin-orbit coupling and suitable band gap,which provides good conditions for its application in spintronics.By doping various transition metal atoms(Cr,Mn,Fe,Co,and Ni),charge transfer occurrs between the PdSe₂ system and the doped atoms,and the redistribution of charges leads to the appearance of new electronic states.Doped with different atoms,the systems show different degrees of spin polarization.When doped Cr atom,the most charge transfer occurres and the spin polarization is the most stable.In addition,the system possess both semiconductor and magnetic properties.These results indicate that two-dimensional PdSe₂ can expected to be a candidate for spintronics related devices.3.Valley polarization properties of two-dimensional Janus WSSe with transition metal atoms(Ti,V,Cr,Mn,Fe and Co)doping and adsorption.The V atom-doped WSSe can obtain the biggest valley polarization.While the impurity levels introduced by other doping atoms hybridize with the valley of WSSe,destroy the integrity of the energy valley.Since WSSe has two different surfaces,S and Se,we compare the two cases of Cr atom adsorption.It can be found that when Cr atom adsorbs on the S surface,the interaction between Cr atoms and WSSe is strong.Hence,the orbital hybridization between Cr atom and PdSe₂ hinders the valley polarization of WSSe.When Cr atom adsorbs on the surface of Se,a huge and stable valley polarization can be obtained.4.Effects of strain on the valley polarization properties of WTe₂ doped with transition metal atoms(Sc,Ti,V,Cr,Mn,Fe,Co and Ni).Doping Co atoms can obtain the largest and the most stable valley polarization in WTe₂.Valley polarization is sensitive to biaxial strain,within an appropriate strain range,the valley polarization increases with the increase of tension force and decreases with the increase of compression force.In addition,under the strain,large valley polarization can be simultaneously obtained both in the valence and the conduction band.By injecting charge and hole carrier into the system,two valley currents with opposite directions can be obtained under the action of external electric field.Based on this,an efficient and low energy consumption spin valley Hall device is proposed.5.Valley polarization and magneto-optical properties in Junas TiBr I/CrI₃and conventional non-polar TiX₂/CrI₃ heterostructures.A giant valley polarization is found and it is attributed to the large charge transfer between TiXY and CrI₃ and their strong hybridization.The magnitude of valley polarization can be manipulated by different stacking and the polarity of TiXY crystal structure.Remarkably,an unusual switch of spin-band(spin-up and spin-down)ordering of Ti Br I can be observed.It occurs not only in conduction band,but also in valence band.The change of spin states in the ground states will bring about a switch of excitons characteristics,such as bright to dark,which can open a novel avenue for spintronics and nano-optics applications.By calculating optical,electronic structure,spin polarization and valley polarization properties of several two-dimensional materials via the first principles calculations,the excellent properties of two-dimensional materials are theoretically predicted and regulated by various strategies.The existance of defects make the mediate states appear in the band gap,which directly leads to the broadening of the optical absorption range.Doping and adsorbing transition metal atoms can make two-dimensional materials obtain spin polarization,and the materials can maintain both semiconductor and magnetic properties.Moreover,doping and adsorbing transition metal atoms and constructing two-dimensional magnetic heterostructures can make the two-dimensional materials obtain valley polarization.These results will provide theoretical help for the related research of two-dimensional materials.